What Is the Previous Period For Google Webmaster Tools Reports?

In October of last year, Google improved the search queries report by adding percent changes to the Google Webmaster Tools report. So if you want to see if you are improving in your rankings, click throughs and impressions, you can do so with this report.

But webmasters are asking in a WebmasterWorld thread, how is the previous period defined?

One would assume that if you put a 30 day date range in the calendar, the change rates displayed in the report would be from the previous 30 days data.

In fact, the report help document says, "By default, we'll show the last 30 days of data, along with how the daily average for the selected period compares with the daily average for the previous period." So as you can see, it doesn't specifically say what happens if you change the last 30 days to something else.

But one webmaster tested it and said if you change your date range to just two days, it will use the previous two days for the change data. He said he was able to test this with a site that has very little traffic.